Qt6 port:

* QApplication::setFallbackSessionManagementEnabled has been removed
* QString::medRef() has been removed. Use QString::mid() again.
* QTextStream::setCodec has been removed
* Use operator QVariant of the QFont class to make code Qt5 and Qt6 compatible
* Signature of QTreeWidget::mimeData() has changed in Qt6. Remove TreeWidget::mimeData() because it doesn't change the implementation
* QLayout::setMargin() is deprecated in Qt5 and has been removed in Qt6. Use QLayout::setContentsMargins()
* QDateTime::toTime_t() is deprecated in Qt5 and has been removed in Qt6. Use QDateTime::toSecsSinceEpoch()
* QDesktopWidget is deprecated in Qt5 and has been removed in Qt6. Use QScreen

src/Gui/Thumbnail.cpp

@@ -116,7 +116,7 @@ void Thumbnail::SaveDocFile (Base::Writer &writer) const
 
     if (!px.isNull()) {
         // according to specification add some meta-information to the image
-        uint mt = QDateTime::currentDateTimeUtc().toTime_t();
+        qint64 mt = QDateTime::currentDateTimeUtc().toSecsSinceEpoch();
         QString mtime = QString::fromLatin1("%1").arg(mt);
         img.setText(QLatin1String("Software"), qApp->applicationName());
         img.setText(QLatin1String("Thumb::Mimetype"), QLatin1String("application/x-extension-fcstd"));